Filter devices for capturing thrombus in the inferior vena cava, commonly referred as “vena cava filters,” are typically delivered to the inferior vena cava by either a femoral or a jugular approach using a catheter to traverse the vasculature and deploy the filter. Vena cava filters are typically deployed infrarenaly within the inferior vena cava, but may be deployed suprarenaly as well.
Vena cava filters typically fall into two general classes: permanent and temporary. Permanent vena cava filters are deployed in such a manner as to engage the vascular wall of the inferior vena cava such as by embedding barbs into the tissue followed by subsequent release from the delivery catheter. Temporary vena cava filters typically are deployed and expand against, but do not permanently embed themselves in the vascular tissue, thereby facilitating removal. Temporary vena cava filters are usually released from the delivery catheter, then later retrieved by using a retrieval catheter that shares or otherwise engages the temporary vena cava filter and collapses it for withdrawal using the catheter.
The accepted standard of care for patients with venous thromboembolism (VTE) is anticoagulant therapy. Inferior vena cava (IVC) filters are reserved for those patients who fail anticoagulant therapy, or have a complication or contraindication to anticoagulant therapy. Until the early 1970's, the only method of IVC interruption was surgical, either by clipping, ligation or plication. The first clinical experience of an endoluminally-placed device to interrupt IVC flow was reported by Mobin-Uddin et al. in 1969. However, it was not until the introduction of a stainless steel umbrella-type filter by Greenfield et al. in 1973 that an effective method of endoluminally trapping emboli while simultaneously preserving IVC flow became possible. Indeed, for many years, the Greenfield filter set a benchmark by which newer filters were measured. Early generations of filters were inserted by surgical cut-down and venotomy. Eventually filters were able to be inserted percutaneously: initially through large 24 Fr sheaths, though newer generations of filters are able to be delivered through 6 Fr systems.
Despite the safety and efficacy of modern day filters, systemic anticoagulation remains the primary treatment for VTE. Either unfractionated or low molecular weight heparin followed by three months of oral anticoagulation in patients with proximal deep venous thrombosis (DVT) is approximately 94% effective in preventing pulmonary embolism (PE) or recurrent DVT. The routine placement of IVC filters in addition to anticoagulation in patients with documented DVT was investigated by Decousus et al. in a randomized trial. Decousus H, Leizorovicz A, Parent F, et al. A clinical trial of vena caval filters in the prevention of pulmonary embolism in patients with proximal deep-vein thrombosis. N Engl J Med 1998;338:409-415. This study revealed that the use of a permanent filter in addition to heparin therapy significantly decreased the occurrence of PE within the first 12 days compared to those without a filter. However, no effect was observed on either immediate or long-term mortality, and by 2 years, the initial benefit seen in the group of patients with filters was offset by a significant increase in the rate of recurrent DVT.
Despite the efficacy of anticoagulant therapy in the management of VTE, there are certain situations and conditions in which the benefits of anticoagulation are outweighed by the risks of instituting such a therapy. These include contraindications and complications of anticoagulant therapy. In such circumstances, there may be absolute or relative indications for filter insertion
Currently, there are at least eleven types of permanent cava filters that are FDA approved. These include the Bird's Nest filter (Cook Incorporated, Bloomington, Ind.), Vena Tech LGM filter (B. Braun, Bethlehem Pa.), Vena Tech LP (B. Braun), Simon Nitinol filter (Bard, Covington, Ga.), Titanium Greenfield filter (Boston Scientific, Natick Mass.), Over-the-Wire Greenfield filter (Boston Scientific), TrapEase filter (Cordis Corp.), the Günther Tulip filter (Cook Inc.), the Cook Celect filter, the Bard Eclipse filter, and the Bard G2X filter.
Well-founded concerns over the long-term complications of permanent IVC filters, particularly in younger patients in need of PE prophylaxis with a temporary contraindication to anticoagulation, has led to the development of temporary and retrievable filters. Temporary filters remain attached to an accessible transcutaneous catheter or wire. These have been used primarily in Europe for PE prophylaxis during thrombolytic therapy for DVT. Currently these devices are not approved for use in the United States. Retrievable filters are very similar in appearance to permanent filters, but with modifications to the caval attachment sites and/or hooks at one end that can facilitate their removal. Retrievable filters that are currently available in the United States include the GÜNTHER TULIP or CELECT Filter (Cook Inc.), OPT EASE (Cordis Corp.), and RECOVERY, G2X or ECLIPSE nitinol filters (Bard Peripheral Vascular, Tempe, Ariz.). The time limit of retrievability is in part dependant on the rate of endothelialization of the device, which typically occurs within 2 weeks. However, differences in design may extend the time period in which the filter may be safely retrieved.
Currently no consensus exists as to which patients have an indication for a retrievable filter. However, it is generally accepted that patients at high risk for pulmonary embolism or with documented PE and with a temporary contraindication to anticoagulation are candidates.
Certain circumstances preclude the placement of a filter in the infrarenal IVC. This includes thrombus extending into the infrarenal IVC, renal vein thrombosis or pregnancy. The safety of suprarenal placement of IVC filters is well documented, with no reported instances of renal dysfunction and no differences in the rates of filter migration, recurrent PE or caval thrombosis.
The rate of upper extremity DVT is on the rise. This is predominantly due to an increasing number of patients having short- and long-term upper extremity central venous access catheters. In one study, 88% of patients found to have an upper extremity DVT had a central venous catheter present at the site of thrombosis at the time of diagnosis or within the previous two weeks. Pulmonary embolism may complicate upper extremity DVT in 12-16% of cases. In patients who have such a complication or contraindication to anticoagulation, a filter can be safely placed immediately below the confluence of the brachiocephalic veins. However, misplacement of an SVC filter is theoretically more likely than with an IVC filter because of the relatively short target area for deployment.
The most common imaging modality used for filter insertion is fluoroscopy, performed either in an interventional suite or an operating room. Bedside placement of filters has inherent advantages, particularly for critically ill patients in intensive care settings where transport can be avoided. Portable fluoroscopy, surface duplex ultrasound and intravascular ultrasound (IVUS) have all been used to assist with bedside filter placement.
Vena cava filter placement frequently occurs concomitantly with central access line placement or in critically ill patients that already have a central access line in place. Heretofore, however, there have been no devices which combine the function of a central access catheter and a removable vena cava filter.
One issue with all vascular filters is the problem of captured clot management. Captured clots raise a risk of total caval occlusion and patient death. Thus, reduction of clot size prior to filter removal or complete thrombolysis of the clot is necessary to restore caval flow patency. Similarly, where a vena cava filter is significantly occluded with clots, removal may be the primary and desired manner of clot management. During filter removal, any captured thrombus is typically squeezed by the contracting structural members of the vena cava filter. The pressure exerted by the vena cava filter on the clot may result in extrusion and or fragmentation of the clot material through the vena cava filter and into the distal blood flow. The attendant risk to the patient of clot material passing through the vena cava filter may be reduced by providing further distal or secondary protection to capture thrombus which is released from the vena cava filter as it is being collapsed and removed.
One particular type of vena cava filter is particularly well suited for use as distal or secondary protection. The BIRD′S NEST FILTER (Cook Medical, Inc., Indianapolis, Indiana) (“BNF”) has some properties which are particularly useful as either secondary distal protection for primary vena cava filter removal or as a vena cava filter suitable for delivery through an already placed central line catheter. The BNF is constructed of a network of four biocompatible stainless steel wires. Each wire is 25 cm long and 0.18 mm in diameter. The wires are preshaped with many non-matching bends of a short radius. The wires are fixed at each end to V-shaped struts, the two legs of which are connected at a junction at an acute angle. A hook with a small loop stop minimizes the risk of IVC perforation at the end of each strut.
When a BNF is deployed, one of the V-shaped paired struts is pushed gently to engage to the IVC wall. Originally, it was recommended that the catheter be withdrawn by 1-3 cm over the pusher wire after the hooks were fixed to the IVC; later, it was recommended that 2 or 3 twists of 360° be applied. The purpose of the twists is to prevent or reduce the chance of wire prolapse. The second pair of struts is then pushed into the IVC so that the junctions overlap by 1-2 cm. The handle of the pusher wire is turned counterclockwise 10-15 times to free it from the struts.
Another wire-type filter is the SAFEFLO vena cava filter (Rafael Medical Technologies). The SAFEFLO vena cava filter, which is described in U.S. Pat. No. 6,482,222 consists of a single continuous wire member having a pre-set expanded shape consisting of plural substantially co-planar radially extending petals and plural ring structures about the circumference of the petals.
A particularly advantageous aspect of the BNF which lends itself to use with the present invention lies in the BNF's use of a network of biocompatible wires which are each pre-shaped with non-matching short radius bends that, upon deployment, form a highly tortuous network of wires which traverse the entire transverse cross-section of the inferior vena cava. Unlike the BNF, however, the present invention employs a conceptually similar, but structurally distinct, filter concept of a single pre-shaped wire or network of wires having a common longitudinal axis, which are deliverable through a lumen of a delivery catheter, such as a guidewire lumen, and upon existing a distal end of the delivery catheter lumen, the wire or network of wires assumes its pre-set shape, which may consist of non-matching short radius bends or other pre-set shapes that cause the wire or network of wires to bend in a manner that traverses across the entire transverse cross-section of the inferior vena cava.